The present invention relates to the direct measurement of density altitude and more particularly to a method and apparatus for directly measuring density altitude.
As is known, the conventional altimeter is basically nothing more than an aneroid barometer having a display face which is graduated in feet of altitude instead of in inches of mercury. Inasmuch as the altimeter is essentially a barometer it provides an indication of altitude which corresponds to the existing atmospheric pressure (ambient conditions of pressure) surrounding the aircraft. Altimeters are customarily calibrated based upon the International Standard Atmosphere which is premised upon a sea-level pressure of 29.92 inches of mercury and a temperature of 59.degree. F. Calculators are available which, by manipulation, take into account a temperature change of 2.degree. C. per thousand feet of altitude change. In such manner temperature compensation for variation in altitude may be introduced. Further, it is conventional to provide for changes in barometric pressure between locations along the flight route by adjustment of the display face, i.e. the Kollsman window to the barometric pressure of the geographic location where the reading is to be made. The altimeter will then provide a reading on its face which is commonly referred to as indicated altitude. However, such reading does not accurately reflect the temperature conditions at the specific altitude and location where the reading is taken nor does it take into account conditions of relative humidity. Temperature and humidity are extremely important factors to be considered since changes in humidity alone can have a significant influence upon the performance of the aircraft. In order to obtain the density altitude it has been conventional heretofore to first obtain an irdication of pressure altitude and then resort to manual computation, charts or complex auxiliary equipment to secure the density altitude.
It has been ascertained, for example, that the hovering ceiling of a helicopter having a gross weight of 1600 pounds changes from 3000 feet under conditions of dry air to only 1300 feet when the atmosphere has changed to 80% relative humidity. This is attributable solely to the variation in the air density due to the increase in humidity. Thus, a helicopter maintaining sufficient lift at cruising speed could have insufficient power to hover to a landing under conditions of high density altitude, and would instead be compelled to make a running landing so as to utilize effective transitional lift. On the other hand, a fixed wing aircraft may be able to utilize ground effect during takeoff but may encounter difficulty in clearing obstacles normally surmounted because high density altitude conditions prevent the aircraft from establishing a safe angle of climb for its gross weight even with maximum power.
It will thus be seen that an indication of pressure altitude alone is not sufficient to provide the pilot with adequate information upon which to make decisions affecting takeoff, landing and cruising of the aircraft. Without an accurate indication of density altitude there is an ever present risk of accident. It is estimated that a significant number of accidents each year are directly attributable to either a lack of information concerning density altitude or an inability to secure such information quickly and easily. Even those charts and apparatuses available heretofore for the determination of density altitude frequently made no provision for changes in humidity and took into account only barometric pressure and temperature.
In view of the foregoing it is one object of this invention to provide apparatus for directly measuring and displaying density altitude taking into consideration the factors of pressure, temperature and humidity.
It is another object of the invention to provide apparatus of the character described which is simple in construction and rugged such that it can be installed in an aircraft and be capable of reliable performance while being subjected to the normally contemplated flight conditions of the aircraft.
It is yet another object of the invention to provide a method for directly measuring density altitude and for providing a direct readout thereof.
Other objects and advantages of the invention will become readily apparent to persons skilled in the art from the ensuing description.